a) Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a semiconductor device including an exposure process of a resist layer covered with a top antireflection coating or film.
b) Description of the Related Art
Highly integrated semiconductor devices have necessarily fine semiconductor elements formed therein. For improving resolution, an exposure wavelength used for photolithography patterning is becoming shorter. If the layer under a resist layer has a high reflectivity, the intensity of exposure light being incident on and reflected from the underlying layer during photolithography becomes high and the intensity of light internally reflected when it reaches the resist layer surface is also not negligible. If there are many kinds of light (incident light, reflected light, multi-reflected light) in a resist layer, this layer is exposed in accordance with the intensity distribution of composite light of all light components.
If an underlying substrate has a step structure, the thickness of a resist layer formed thereon changes so that the phase and intensity of reflected light in the resist layer vary. The distribution of amplitudes of standing waves in the resist layer therefore varies and the precision of resist pattern size lowers.
If the thickness of a resist film changes because of an uneven surface of the underlying substrate, the amplitudes of standing waves in the resist layer varies and the resist pattern precision lowers. In such a case, if reflection at the resist layer surface is reduced, the intensities of standing waves in the resist layer may be made uniform.
Basing upon this concept, techniques of forming an antireflection film on the surface of a resist film have been proposed. Such an antireflection film is called a top antireflection coating or film. For example, the top antireflection film is formed on the surface of a resist layer to a thickness of (.lambda./4n.sub.a) by using material having a refractive index n.sub.a lower than the refractive index n.sub.r of the resist layer. This layer having a thickness of (.lambda./4n.sub.a) is a .lambda./4 plate having an optical length of .lambda./4 in the thickness direction (in this specification, an optical length may also be called an optical thickness). The .lambda./4 plate is used as a single layer antireflection film, and shows the maximum effects of antireflection at n.sub.a =(n.sub.r).sup.1/2.
Studies made by the present inventors indicate that even if a top antireflection film is formed on a resist layer to an optical thickness of .lambda./4 by using material having a refractive index smaller than that of the resist layer, the resist pattern size precision is not necessarily improved, but it may lower in some cases. If the underlying layer of the resist film is a transparent film in particular, the resist pattern size precision becomes likely to be worsened.